Magnetic Properties of Some Iminoxyl Polyradicals. V. EPR Studies of the TEMPAD Biradical

Abstract

The EPR of pure and diluted crystals of bis(2,2,6,6-tetramethylpiperidine-4)azine-1,1′-dioxyl (the TEMPAD biradical) was studied. From the spectra of the diluted cyrstal, the intra- and inter-molecular dipole-dipole interactions were identified. The crystal-field parameters of the intra-molecular coupling determined for the EPR spectra were: |D|⁄hc\simeq0.00477 cm⁻¹ and |E|⁄hc\simeq0.00032 cm⁻¹ and two kinds of principal axes were found. As the z-axis of the intra-molecular dipole-dipole coupling is in the direction connecting two N–O bonds in a molecule, the TEMPAD molecules in a unit cell may be said to occupy two different sites. The crystal-field parameters of the inter-molecular dipole-dipole coupling were determined to be: |D′|⁄hc∼0.012 cm⁻¹ and |E′|⁄hc∼0.003 cm⁻¹ and the z-axis of the tensor was in the direction of the a-axis. The assumed molecular-stack model could account for the aforementioned results. The EPR spectra of the pure TEMPAD biradical are one symmetrical Lorentzian line in all the directions; the angular dependence of the peak-to-peak linewidths was explained by means of the term of the inter-molecular dipole-dipole interaction obtained from the spectra of the diluted crystal using the three-dimensional Anderson-Weiss formula. The exchange parameter for the best-fit curve was of the order of magnitude of the weak exchange interaction estimated roughly from the small Weiss constant appearing in the paramagnetic susceptibility of the diluted crystals.